This invention relates to means for sensing certain characteristics of the transistors of a circuit and for using information relating to the characteristics for controlling the operation of the circuit.
The design of many circuits and their proper operation depends on the components of the circuits having certain characteristics. For example, an output driver may include a first pull-up transistor connected betweeen a first power supply terminal and a load terminal and a second pull-down transistor connected between the load terminal and a second power terminal. Normally, the pull-up and pull-down transistors are designed to be turned on at different times whereby when the pull-up transistor is turned-on, the pull-down transistor is turned-off and when the pull-down transistor is turned-on, the pull-up is turned-off. In addition, in the design of the output driver, it is assumed that the transistors have a certain conductivity when they are turned-on in order to drive the output terminal high or low within a preset period and at a predetermined current level with a given load.
When the characteristics of the actual transistors used in the circuit vary significantly from the assumed values, significant errors and problems arise in the operation of the circuit. Three factors which may significantly affect the characterisitics of the transistors of a circuit are:
(1) process variation; PA1 (2) power supply variation; and PA1 (3) temperature variation.
The problem with process variation is that the components formed during one run may have significantly different characteristics than those formed during a subsequent run, even though both runs use the same process steps and are carried out under a very similar environment. For example, the main conduction paths of transistors formed during one run may have a lower equivalent impedance (higher conductivity) than those formed during another run. Transistors with a lower equivalent impedance would have a higher conductivity and be able to produce more current for the same bias condition. Consequently, these transistors appear to be faster since they could charge or discharge a node faster than transistors having a higher equivalent impedance.
The value of the operating potential applied to the transistors of a circuit also has a significant effect on the characteristics of the components of the circuit. For example, as the voltages across the different terminals of a Metal-Oxide-Semiconductor (MOS) transistor increase, the conductivity and effective speed of response of the MOS transistor increase, since the transistor can supply more current.
With regard to temperature variation, as the ambient temperature increases, MOS transistors exhibit a higher equivalent impedance (lower conductivity) whereby they pass less current and are effectively slower than at lower temperature.
Referring, for example, to the output driver circuit, the variations in the characteristics of the transistors constituting the circuit may be such that when the transistors of the circuit exhibit a high conductivity, the speed of response is such that very large currents flow along the power supply lines generating large negative voltage drops (glitches) along the positive power supply lines and large positive voltage drops along the negative or ground return lines. The glitches produce a whole range of noise signals which may be coupled to the driver circuit and other circuitry formed on the same integrated circuit as the output driver circuit. For example, a threefold increase in the speed of response of the transistors causes a ninefold increase in the magnitude of the glitches [where the glitches are caused by both the speed of response of the driver transistors and the rate of change of their input signals]. Also, when the transistors of the circuit have high conductivity, there is a greater tendency for the pull-up and pull-down transistors of the circuit to be turned-on at the same time causing spike through (i.e. large current spikes passed via the transistors between the positive and negative supply line).
On the other hand, if the pull-up and pull-down transistors exhibit low conductivity and appear to be very slow, then the signal generated at the output terminal (and/or to the load connected thereto) may not be able to drive the output terminal to a desired level within a desired period of time for a given load. This may result in the production of erroneous signal information at the circuit output.
In practice, a designer will design a circuit taking into account the worst case condition. For purpose of illustration and using MOS transistors as an example, the worst case condition would be the one for which the process is "slow", the operating potential is low and the temperature is high. Thus, in the design of a driver circuit, pull-up and pull-down driver transistors would be sized to provide the needed load currents under the worst case condition. In addition, circuitry controlling the driver transistors would be designed to turn them on as quickly as possible. As a result of this design, if there is any improvement in process speed, operating potential or temperature, the drivers will turn-on faster and harder causing larger than needed currents to flow with the concurrent production of noise signals. Under the best condition of process, potential and temperature, the circuit would be significantly over-designed and produce large glitches and noise signals.
In brief, the problem to be resolved is the design of a circuit which operates within a set of specifications under the most extreme conditions. For the case of a driver circuit employing MOS transistors, one extreme condition (high conductivity/high speed of response) occurs when the process is "fast", the power supply is "high" and the temperature is "low"; and the other extreme condition (low conductivity/low speed of response) occurs when the process is "slow", the power supply is "low" and the temperature is "high".